An eye diagram corresponds to a superposition of samples of a serial data signal over a unit interval of the data signal (i.e., the shortest time period over which the data signal can change state). An eye diagram may be generated by applying the serial data signal to the vertical input of an oscilloscope and triggering a horizontal sweep across the unit interval based on the data rate of the serial data signal. When the serial data signal corresponds to a pseudorandom data signal, the superposed samples appear on the oscilloscope display as an eye diagram with an eye opening bounded by two transition regions. Various features of the eye opening reveal information about the quality of the communications channel over which the serial data signal is transmitted. For example, a wide eye opening indicates that the serial data signal has a relatively low noise level and a relatively low bit error rate, whereas a narrow eye opening indicates that the serial data signal has a relatively high noise level and a relatively high bit error rate.
An eye diagram typically is evaluated based on the width of the eye opening, the height of the eye opening, and the rate of closure of the eye opening with variation of the sampling time. The width of the eye opening corresponds to the time interval over which the serial data signal can be sampled without inter-symbol interference. The height of the eye opening corresponds to a measure of the signal-to-noise ratio of the serial data signal. The rate of closure of the eye opening with variation of the sampling time indicates the sensitivity of the serial data signal to timing errors.
Various eye monitor circuits have been developed that measure one or more characteristic features of an eye diagram of a serial data signal in real-time. The measured features typically are used to correct distortions that are introduced into the serial data signal by the communication channel. For example, the frequency responses of some adaptive equalizers are optimized based on measurements of current signal quality as indicated by one or more characteristic eye diagram features.
The cost and power dissipation requirements of eye monitor circuits are dominated by the number of high-speed hardware components in the eye monitor circuit. Consequently, eye monitor circuits that have fewer high-speed hardware components provide a strategic advantage over other eye monitor circuits that have more high-speed hardware components.